1. Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to a turbine blade with a spar and shell construction.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In a gas turbine engine, a compressed air from a compressor is burned with a fuel in a combustor to produce a hot gas flow. The hot gas flow is passed through a multiple stage turbine to convert most of the energy from the gal flow into mechanical work to drive the compressor, and in the case of an aero engine to drive a fan, and in the case of an industrial gas turbine (IGT) engine to drive an electric generator to produce electrical power.
The efficiency of the engine can be increased by passing a higher temperature gas into the turbine, or a higher turbine inlet temperature. However, the maximum turbine inlet temperature will depend upon the material properties of the first stage turbine stator vanes and rotor blades, since these airfoils are exposed to the highest gas flow temperature. Modern engine has a turbine inlet'temperature around 2,400 degrees F., which is much higher than the melting point of a typical, modern vane or blade. These airfoils can be used under these high temperature conditions due to airfoil cooling using a mixture of convection cooling along with impingement cooling and film cooling of the internal and the external surfaces of these airfoils.
A few very high temperature materials exist that have melting points well above modern engine turbine inlet temperatures. Columbium (or Niobium) has a melt temperature of up to 4,440 F; TZM Molybdenum up to 4,750 F; hot pressed silicon nitride up to 3,500 F; Tantalum up to 5,400 F; and Tungsten up to 6,150 F. these materials would allow for higher turbine inlet temperatures. However, these materials cannot be cast or machined to form turbine airfoils.
On prior art method of forming a turbine airfoil from one of these exotic high temperature materials is disclosed in U.S. Pat. No. 7,080,971 B2 issued to Wilson et al on Jul. 25, 2006 and entitled COOLED TURBINE SPAR SHELL BLADE CONSTRUCTION, the entire disclosure being incorporated herein by reference. The shell is formed from a wire EDM process to form a thin walled airfoil shell, and the shell is held in compression between a spar tip and the blade platform or root section. The shell can take the higher gas flow temperatures, and the spar provides internal cooling for the airfoil walls.
Part life is another important factor in the engine, especially for an industrial gas turbine (IGT) engine. In a spar and shell turbine blade, the spar is held in place between the blade platform or root section and the spar tip. The shell in the Wilson et al (U.S. Pat. No. 7,080,971) would be placed under a compressive load due to the centrifugal force acting to push the shell in the blade radial outward direction and up against the underside of the spar tip edge. Operating the shell of the turbine blade with, a spar and shell construction is better than operating the shell under a tensile loading because the tensile loads will have a shorter life than one under a compressive loading. Operating the shell under near zero loading would allow for an infinite life for this part since the part would be operating under practically no loading. Except for the thermal loading and pressure loading from the hot gas flow reacting onto the shell surface, the turbine blade could be used for an indefinite period of time.